Retzuot (Tefillin straps) need to be checked periodically. The top side must be totally jet-black. If the color is flawed, use this convenient easy brush applicator Tefillin Ink in a matte finish to do a touch-up job. The laws of producing the ink are intricate. It is important that all inks carry a reputable Kashrut seal. This ink is “Mehadrin Kosher.”